sub _sort_column {
        my $key = shift;
        my $array_ref = [];

        if( $key eq 'location' ) {
            # location is a pseudo-column,
            # containing these columns:
            $array_ref = [
                'astro.galaxy',
                'astro.region',
                'astro.system',
                'astro.astro',
            ];
        }
        else {
            $array_ref = [ $key ];
        }
        return $array_ref;
    }